1. Field of the Invention
The present invention relates to an inverter circuit for lighting discharge lamps for use in a liquid crystal display unit, and the like, and particularly to an inverter circuit with a high power efficiency.
2. Description of the Related Art
In some conventional inverter circuits for lighting discharge lamps, a resonant circuit may be formed by leakage inductance at the secondary side of a transformer and by parasitic capacitance in a discharge lamp connected as load, and the primary side of the transformer may be driven by a resonant frequency of the resonant circuit thus formed. An example of such inverter circuits is disclosed in U.S. Pat. No. 6,114,814. Such a conventional inverter circuit to drive the primary side by the resonant frequency involves phase difference in voltage and current at the primary side of the transformer consequently failing to achieve a favorable power efficiency.
In order to cope with the problem described above, Japanese Patent Application Laid-Open No. 2003-168585 discloses an inverter circuit for discharge lamps, in which a transformer is driven in a frequency range where phase difference in voltage and current at the primary side of the transformer is small thereby providing a high power efficiency, whereby the power efficiency of the transformer is improved. The inverter circuit for discharge lamps disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2003-168585 comprises: a transformer where a resonant circuit is formed by parasitic capacitance in a discharge lamp and auxiliary capacitance; and an H-bridge circuit where the primary side of the transformer is driven at a frequency which is less than a series resonant frequency of the resonant circuit, and at which phase difference in voltage and current at the primary side of the transformer falls within a predetermined range from its minimum, thus the power efficiency is improved.
In an inverter circuit for discharge lamps used in a liquid crystal television (TV) which is one example of liquid crystal display (LCD) units, a supply voltage ranges from 12 to 24V. For example, in a separate driving inverter which is described in connection with the aforementioned inverter circuit disclosed in Japanese Patent Application Laid-Open No. 2003-168585, and which uses a leakage magnetic flux type transformer, an inverter control IC to constitute a control section of the inverter circuit is operated by a supply voltage of 5.0V, and an H-bridge circuit with an FET to drive a transformer thereby lighting discharge lamps is operated by a voltage of 12 to 24V.
Recently, a liquid crystal TV is increasing in its screen size, and as many as 8 to 24 discharge lamps are employed in one liquid crystal TV, and also the length of discharge lamps is increased to, for example, 1300 mm. This results in increasing the power consumption up to 180 W. Accordingly, in case of a large-sized liquid crystal TV, its inverter circuit and its discharge lamps are responsible for most of the power consumption, and therefore the inverter circuit is required to be further improved in efficiency for reducing its power consumption.
In order to answer the above-described requirement for improved efficiency of an inverter circuit for discharge lamps, there is provided an inverter circuit in which a voltage supplied to the H-bridge circuit for lighting discharge lamps is increased from conventional 12 to 24V up to, for example, 120V. Since, current flowing in the FET can be reduced due to the increased supply voltage in the inverter circuit, loss due to on-resistance of the FET can be reduced, and also since current flowing in a primary winding of a transformer can be reduced, copper loss can be reduced. Thus, its efficiency is improved. Here, two supply voltages are involved: one is 120V supplied to the H-bridge for lighting the discharge lamps, and the other is 5V supplied to the inverter control IC. The withstand voltage of the FET of the H-bridge must be increased, and a high gate source voltage is required to drive the FET with a high withstand voltage. For example, if the withstand voltage of the FET of the H-bridge is set at 200V, the gate source voltage of the FET of the H-bridge must be 10V or higher. Consequently, the FET cannot be driven by a voltage of 5V supplied to the inverter control IC if used as it is supplied, and the voltage supplied must be stepped up by a discharge pump, a bootstrap, or a step-up DC-to-DC converter in order to duly drive the FET.
However, employing a step-up circuit like the aforementioned discharge pump, bootstrap, or step-up DC-to-DC converter complicates the circuit structure and increases the number of components. Also, another problem is that there is difference between frequency of an oscillating circuit to operate the H-bridge circuit and frequency of another oscillating circuit to operate the step-up circuit, which produces interference at a reference voltage of the inverter control IC thus interrupting a stable operation of the circuit.